Spark plug tester



July 13, 1943. M. F. PETERS ETAL SPARK PLUG TESTER Filed March 18, 1942SPARK P1.ue*2

SPARK PLUG INVENTORS.

ATI'ORNEY cause of theiact that Patented July 13, 1943 P RK r E TEMelville .Peters, Belt .Garrett Park, Md., and

ton, no.

sville, and, John PrBoston, Merlin Davis,.washing- Application March 18,1942, Serial No. 435,192

" "14 Claims. (or 175-183 (Granted under the act of March 3,

e amended April 30.1928; 370 0. G. This invention relates to a new andimproved method and apparatusfor measuring the resistance of a sparkplug for an internal combustion engine, and more particularly to theimproved method and apparatus for obtaining a substantiallyinstantaneous measurement of the resistance of a spark plug when itisinstalled in an operating internal combustion engine, and at aparticular point in the operation cycle of its respective cylinder.v V

Thefresistance of a spark plug must be equal to or greater than 175,000ohms, otherwise the spark plug would misfire when used with the presentignition generating apparatus, therefore it is important to haveavailablea method for quickly and easily obtaining an accurate measure.-ment of the resistance of a spark plug. Furthermore, the value of thespark plugs resistance maybe used as an index of the spark plu'gscondition. It is well known that after a spark plug 2 has been in usefor some time the insulation between the spark gap electrodes becomescoated or impregnated with carbonaceous and lead deposits and that thesedeposits which are conductive may in time form an electricallyconductive path between the electrodes which will serve to carry acurrent sufllcient to prevent the voltage impressed across the gap frombuilding up to a value equal its break down potential. This conductivepath formed onthe dielectric of the plug causes the spark plug stalledin an engine. tore, that a measure to misfire when it i in- It should beapparent, thereof the spark plug's resistance would serve as a usefulindication of thecondition of the insulation in determining whether ornot the plug is faulty and should be cleaned or replaced. I

The resistance ofaspark plug i not constant but changes with variationsin temperature. Con- I sequently the resistanc measurements, to beuseful as an index ofthe spark plug's condition,

temperature of the plug must be taken when the is equal to its normaloperating temperature. Be-

the temperature gradients in a spark plug vary plug, it issubstantiallyimpossible to determine the temperature to which any part of the plugmust be heated when it is not installed in an operating intemalcombustion engine in order to approximate the temperature it wouldacquire wheninstalled in an operatingcylinder of an internal combustionengine. It is desirable, therefore; to measure the resistance of thespark plug when it is installed in an operating cylinder of aninternalcombustion engine.

in different parts of the In obtaining the resistance measurements ofspark plugs when they are installed in: an operating internal combustionengine, it is essential that reoccurring resistance measurement be takenat substantially the same point in the operatingcycle of the cylinder ofengine into which the the resistance of the spark gap is subject tocyclic variations. For example, the resistanc across the spark gap isquite low when the plug is firin because the spark discharge itselfforms a good electrically conductive path. Furthermore, duringcombustion of the explosive mixture in the cylinder and during exhaustof the gases of combustion, the resistance across the gap is alsorelatively low because the gases of combustion offer a relatively goodconductive path across the l spark gap. Consequently, the resistancemeas-* urements must be taken at some particular point in the operatingcycle of the particular cylinder into which the plug is installed othering ignition, combustion or exhaust. It is preferred in this inventionto make all comparative measurements during the compression stroke andat the same point in the compression stroke of the operating cycle ofthe test plugs respecperature which the plug would attainwwhen' in--stalled in an operating internal combustion engine.

It is also an vide a novel method for obtaining the resistancemeasurements of the spark plug when the spark plug is installed in anoperating cylinder of an internal combustion engine. This method permitsthe operator to discriminate between the low values of resistance acrossthe spark gap which are present during periods of spark discharge,conbustion, expansion,

mainingportlons of the cycle. In this manner the resistance measurementsmay be taken at some point in that portion of the operatingcycle duringwhich the resistance of the spark gap is at its maximum value.

It is a still further object of this invention to provide a. method forobtaining a. rapidly re- 5| curringyet substantially instantaneousindicathe internal combustion plug is installed because than durobjectof this invention, thereobject of this invention to proand exhaust, andthe higher resistance which exists during the reof the spark plug ill ofin the operating cycle of test ylinder.

plug's respective A more specific object of this invention is to knownand constant electroapply periodically a ing said spark gap.

It is also a specific object of the invention to provide a timing devicewhich is utilized in obtaining a rapidly recurring yet instantaneousOther objects and advantages of this invention will become apparent fromthe specification as the specific features of certain preferredmodii'ications are hereinafter described in detail in connection withthe accompanying drawing, in which spark plug'installed inthe sameengine; and Fig. 3 is a schematic 'cir'cuit diagram of still anotherform 01' this invention.

In Fig. 1 of the drawing the particular cylinder oi' the internalcombustion engine into which has been installed'is indicated of thereference numeral it. for this internal secondary is connected to therotor 46 of the distributor 41. This distributor is provided with aplurality of conventional stationary cooperating contacts each of whichis connected to a diilferent spark plug in accordance with' the firingorder of their respective cylinders.

The test circuit for measuring the resistance M on the drawing andconsists specifically of a sensitive ammeter or galvanometer IS, asource of constant direct current potential l6 of a known magnitude, afixed resistance I 1 also of a known value, the spark plug I II and aperiodition or the spark plug's resistance at thesame selected instantI! oi the disc II in the operating portion particular instant cycle ofthe cylinder II at which the resistance measurements are to be taken.

It is thus apparent that the' measurement of the resistance of the sparkinternal resistance. may be readily converted into values of resistanceby utilizing the known values of It and the resistance I! in oi Ohmslaw.

Since the use or the rotary switch I! requires mechanical the grid 33from the source of grid potential 39. Connected directly jecting fromthe enclosing envelope and from the anode 36 and the cathode gas triode34 and in series with the r only at the II 01' this applied tothecontrol grid 33 prevents current flow through the thyratron duringcharging of the condenser 26 connected across the plate circult,thereof. A soon as the ignition potential is applied to plug 32,however, the negative bias is removed and the capacitor 26 dischargesrapidly through the gas triode 34 and loses its potential. As soon asthe condenser voltage drops below the ionization potential of the gas inthe tube 34, the grid 33 regains control and the current through thetube 34 ceases. The condenser may now be again recharged to start a newcycle .of operation since the tube 34 is now non-conductive and acts asa closed control valve in the shunt circuit.

A suitable alternating current supply source is connected to the primary58,01 the transformer 51. One of the secondary windings 59 of thetransformer 51 is connected to supply a heatin current to the cathodesGI and G2, producing a temperature rise therein sufficient for electronemission. The terminals of the secondary winding 3i of the transformer51 are connected to each of the anodes 53 and 64 of the rectifier 2|.and to complete the conventional push pull rectifying circuit themidpoint B of the secondary winding 3| is connected to the other outputterminal comprising the cathodes GI and 62 of the rectifier tube 2ithrough a series circuit which includes the inductive reactance 85 ofthe filter network, the fixed resistors 56 and and the leads 18 and 69.The filter circuit includes in addition to the inductive reactance 65the conventional shuntin condensers 61 and 68.

Thus a substantially constant direct current voltage is continuouslyappliedto the series circuit including the fixed resistors 66 and 20.

Another secondary winding 54 of the transformer 51 is connected tosupply current to a heating circuit for the cathode of the diode tube30. The anode of this diode tube is connected to the same terminal ofthe selective condensers 25 to 29 inclusive to which the cathode of thespace discharge device 34 is connected. This connection is not completedon the drawing but is indicated by the arrows and the potential o 450 v.The cathode of the diode tube,

- or half wave rectifier tube 30 is connected to one terminal of thesecondary winding 3| of the transformer 51 and the conductor 69 leadingfrom the midpoint of this secondary winding 3| completes a seriescircuit shunting one of the selective condensers through the resistanceunits 1| to 15 inclusive of the voltage divider 24 by means of theselective contact arm 16. Thus a charging circuit for the selectivecondensers has been established from the secondary 3| through the diode30 and the voltage divider.

A ground connection 11 is provided for the series resistance 20 throughthe conductive lead 18 and serves to connect the. measuring probe 19 andthe fixed resistance l1 ina series circuit through ground with theresistance IQ of the spark plug l0. Thus the voltage drop appearingacross the resistance 20 which receives the source of energy from thefull wave rectifier 2| isapplied to a series circuit including the fixedresistance l1 and the resistance of the spa.k gap IS. A cathode rayoscillograph tube 80 is provided 1 which has a cathode heating circuit86 adapted to be connected to the terminals of the secondary winding 54.

The cathode 8|, anode 82 and control grids 83, 84 of the cathode raytube 80 are connected to their respective portions of the voltagedivider 24, so that the cathode ray of this tube will be initiated onlywhen the current flowing in the series circuit including the voltagedivider 24 has a magnitude sufflcient to produce a voltage drop in theresistance elements 1| to 15 inclusive equal to that required to intiatethe electron beam when the said voltage is applied to the cathode, anodeand control grids oi the said cathode ray tube. Thus the tube to isocculted the time during which no current flows in the series circuitcontaining the voltage divider 24 and is flashed only during the arequired magnitude flows therein.

For convenience of illustration, a conventional type of cathode raydevice 80 has been illustrated having two sets of deflection plates 22and 23. For the purpose of this invention, however, only a single set ofthese deflection plates are used; consequently the deflection plates 23are shown grounded out by means of the ground connection 81. Theremaining set of deflection plates 22 are connected across theresistance IQ of the spark plug ill in series with the fixed resistor l3so that the potential drop existing across the spark gap will always beapplied to the deflection plates through the fixed resistor l3.

The apparatus disclosed in Fig. 2 operates as follows:

A control plug 32 is selected which will fire at the particular point inthe operating cycle of the cylinder into which the test plug II! isinstalled at which the resistance measurements are desired to be taken.The rectified potential of the secondary winding 3| of the transformer51 is applied to a series circuit including the resistance element 20and causes a substantially constant direct current to flow in thiscircuit so as to produce a substantially constant potential drop acrossthis resistance element 20. This source of constant potential isimpressed across the series resistance l1 and the internal resistance IQof the spark plug Ill. The cyclic change in the resistance 19 of thespark plug Ill causes the potential drop across any portion of theseries circuit including the resistance l1 and the spark plug ID tochange in accordance with the varia-- tions in resistance of the plug.The cyclic variations in the potential drop which occur across theresistance is produce a cyclic variation in the potential applied to thedeflecting plates 22, since the potential drop across the resistor is isapplied to the series parallel circuit including the deflecting plates22, their shunting resistor and the series resistance l3. The resistancel1 is connected in series with the spark gap so that the variations inresistance of the plug it produces variations in the potential divisionof the series circuit including the resistances l9 and I1. That is, wereit not for the resistance l1 the potential drop occurring across theinternal resistance l9 would not change with the cyclic variations ofthe resistance of the spark gap but would at all times equal theconstant potential drop in resistor 20. It is thus apparent that thepotential applied to the deflecting plates 22 is proportional to andtherefore a measure of the resistance 19. While the potential dropacross the spark gap resistance In varies cyclically with the variationsin the resistance of the spark gap and while this potential drop isconstantly impressed across the deflecting plates 22 of t cathode raytube 80, this potential drop is used as a measure of the resistance onlyat the preselected instant or point in the cycle of the plug'srespective cylinder, since the cathode ray tube is time current ofselected point.

- pendent upon the is connected in series With the the gas triode 34.

occulted at all times during the operating cycle of the cylinder l8other than at the particular preselected instant or point. The tube 80is flashed at this point in the operating cycle and the ray isdetectable by noting the formation of a beam spot on the fluorescentscreen of the oathode ray tube 80. The deflection of the beam spot willbe a measure of the plug's resistance at the The cathode ray tube ismade to flash only at the selected instant by making its operationdepresence of a voltage drop in the resistance elements 1| to inclusivewhich form the voltage divider 24. That is, so long as current flows inthe circuit including the voltage divider 24 the cathode ray would beinitiated and maintained. The source of voltage producing the currentflow in this divider is the rectified potential of the transformer 3|.And to make the current fiow intermittent and controllable in accordancewith the cyclic operation of the cylinder l8 into which the test plug isinstalled, there voltage divider 24 one of the selective condensers 26of the group to 29 inclusive, and the half wave rectifier 30. The onlycurrent which can flow in this circuit which includes the voltagedivider is the current necessary to charge one of the selectedcondensers. Each of the condensers have a different capacity rangingfrom one tenth to five thousandths of a microfarad; thus the timeconstant of the series circuit may be changed by the proper selection ofone of the various condensers.

At the end of the period of time required to charge the selectedcondenser 26 the current flowing in the voltage divider 24 ceases,whereupon the potential applied across the cathode 8|, anode 82, controlgrids 83 and 84 of the oscillograph 80 is likewise removed and the beamspot formed by the cathode ray or electron beam striking against thefluorescent screen occults or becomes extinguished due to the lack of asuflicient operating potential from the voltage divider.

Since current flow in the circuit must be made to occur only during theportion of the cycle at which resistance measurements are to be takenthe discharge of the condenser and consequently the flow of a rechargingcurrent therefore is made to depend upon the ignition point in theoperating cycle of another cylinder of the same internal combustionengine, That is, the ignition potential of the spark'plvg 32 isconnected by the capacitive coupling 49 to the control grid 33 of Thistriode 34 when rendered conductive by removal of the negative grid biasforms a low resistance shunt path across the charged condenser 26allowing the condenser to discharge. This discharge takes place during acomparatively short interval after the control plug 32 has fired and thegrid control is restored to the tube lected condenser has dropped to apoint where it is less than the ionization potential of the gas in thespace discharge device 34 and will again gain control to act as a closedcontrcl valve in the circuit including the condenser 26.

The conductance of the path through the space discharge device 34 whenthe same is rendered conductive by the control grid 33 is so greatcompared to the conductance of the path through the half wave rectifierthat after the selected condenser 26 has sufllciently discharged thereis no electron source sufficient to maintain ionization of the gas inthe discharge device and the of a microfarad 34 as soon as the potentialof the se- 1 curring across the been illustrated and flow of current isinterrupted. Substantially immediately upon discharge the selectedcondenser is again recharged through the diode 30 and the circuitincluding the voltage divider 24. the recharging period the flow oicurrent through the voltage divider 24 is again established and thecathode my will be initiated forming a beam spot on the.fluorescent-screen of the oscillograph tube 80. This flash of the beamor ray takes place only during the recharging period of the selectedcondenser 26, and since at this particular instant the potential dropoccurring across the resistance IQ of the plug I0 is applied through theresistance H to the deflection plates 22 of the cathode ray tube thedeflection of the beam since the occurrence of the recharging period ismade to depend upon the ignition potential applied to the control lug32, the flash of the oathode ray tube is also made at substantially thesamieapoint in the operating cycle of the cylinder used to indicate thesame parts of the ignition circuit in each of the various illustratedmodifications. In the modification illustratedby Fig. 3, a source ofconstant electrical potential 38 is connected in series with aresistance 37 and the resistance IQ of the test spark plug ill. Thedeflection plates 22' of the constantly energized cathode ray tube 80are connected across a portion of this resistance element 31. As theresistance of the spark gap changes in accordance with the operatingcycle of the cylinder into which it is installed, the current flowing inthe series portion of the resistance 31 to which the deflection plates22' are connected will be a minimum its maximum value. The comparativevalue of the resistance of the plug may be obtained at the point in thecycle of the cylinder of the internal combustion engine into which thetest plug I0 is installed at which it has its maximum resistance, simplyby noting the minimum deflection of the cathode ray. This minimumdeflection of the cathode ray produced with the test plug installed maybe compared with the minimum deflection obtained when a plug of standardresistance is installed in the engine in order to enable the operator ofthe device to tell at a glance whether or not the resistance is aboveorbelow a fixed acceptable value, such as the value of 175,000 ohmsmentioned above.

According to the tutes the preferred provisions of the patent forms ofmy invention have described, with the understanding, however, thatcertain modifications may be made without departing from the inventionand that the invention may be practiced otherwise than specificallyillustrated and described and still remain within the scope of theappended claims.

Th invention described herein may be manufactured and used by or for theGovernment 01' the United States of America for governmental when theresistance I9 is at.

, responsive to the potential connecting said second eluding a cathoderay tube any royalties described our invention, what cluding a gridcontrolled space discharge device,

means utilizing the periodically developed ignition potential of anotherinstalled and operating spark plug of the same internal combustionengine for rendering said space discharge device conductive, a condenserconnected across the terminals of said space discharge device, a firstsource of unidirectional current, a resistance element, circuit formingmeans connecting said first unidirectional current source to saidcondenser in series with said resistance element to charge saidcondenser, a cathode ray oscillograph, means drop across at least aportion of said resistance element for initiating said cathode ray, asecond source of substantially constant direct current potential,circuit means source of direct current potential to the spark gap ofsaid test plug so that the cyclic change in the resistance of said gapwill cause a corresponding cyclic change in the voltage gap of said testode ray an amount proportional to the said voltage drop, whereby arapidly recurring yet constantly deflected ray will indicate thesubstantially instantaneous resistance of the spark plug at the saidparticular point in the operating cycle of its cylinder.

2. In a system for determining the internal resistance of a spark pluginstalled in an operatin internal combustion engine at a particularpoint in the operating cycle of its respective cylinder, the combinationincluding a source of substantially constant electrical potential, ingmeans for connecting said source of potential to the. spark gap of saidtest plug so that cyclic variations in the resistance of said gapproduce corresponding cyclic variations in the potential drop across theplug, a cathode ray tube, means substantially immediately responsive tothe periodically other spark plug in the same engine for periodicallyinitiating the cathode ray of said tube, and means for deflecting saidray an amount proportional to the potential drop across the spark gap ofsaid test plug whereby the deflection of said rapidly recurring cathoderay will be an indication of the instantaneous value of the resistanceof said plug.

3; In a system for determining the internal resistance of a spark pluginstalled in an operating internal combustion engine, the combinationinhaving means for defleeting said cathode ray, means for controllingthe said cathode ray tube so as to cause the said to be occulted duringa substantial period in the operating cycle of the cylinder of theinternal combustion engine containing the spark plug under test and topoint in the operating cycle of said cylinder other than ignition,combustion, and exhaust, and means for causing the said deflecting meansto deflect said cathode ray an amount proportional to the electricalresistance across the spark plug under test whereby the said deflectedcathode ray developed ignition potential of an can be made to appearduring a selected instant in the cycle of the said cylinder thus givinga rapidly recurring yet instantaneous measurement of the spark plugresistance at the selected instant. I

4. In an electrical system for determining the internal resistance of asp D 8 in ta led in an operating internal combustion engine at aparticular point in the operating cycle or its respective cylinder, thecombination including a source of substantially constant electricalpotential, circuit forming means for connecting said potential to thespark gap of said test plug so that cyclic variations in the resistanceof the spark gap of said test plug produce corresponding cyclicvariations in the potential drop across said spark gap and meansforobtaining a rapidly recurring yet instantaneous indication of thevoltage drop across the spark gap of said test plug at the selectedpoint in the operating cycle of the respective cylinder.

5. In a system for measuring the maximum internalresistance of a sparkplug installedin an operating internal combustion engine, thecombination including a fixed resistance element, an electrical powersupply source of substantially constant potential, circuit forming meansconnecting said resistance element and spark gap of said spark plug tosaid power supply source, a cathode ray tube having ray deflectingplates and means connecting the deflecting plates across at least aportion of said fixed resistance element so that the cyclic variationsin resistance of said plug will cause the potential drop across at leasta portion of said resistance element to change producing variations inthe deflection of said cathode ray, whereby the relative value of themaximum resistance of said spark plug may be determined by noting thedeflection of said cathode ray.

'6. In a system for measuring the maximum internal resistance of a sparkplug installed in an operating internal combustion engine, thecombination including a source of substantially constant electricalpotential, a resistance element,

circuit forming means for connecting said resistance element andpotential source to the spark gap of said test plug so that cyclicvariations in the resistance of the spark gap of said test plug producecorresponding cyclic variations in the potential drop across a portionof said resistance unit and means utilizing the variations in thepotential drop across a portion of said resistance unit for indicatingthe cyclic variations in the resistance oi. said test plug, whereby therelative value of the maximum resistance of said spark plug may bedetermined. 7

'7. In a system for measuring the internal resistance of a spark pluginstalled in an operating internal combustion engine the combinationincluding an electrical power supply source of known potential,,anammeter, and means synchronized with said internal combustion engine-for connecting said power supply source in a series circuit with saidspark gap and ammeter at a particular point in the operating cycle ofsaid engine other than during ignition, combustion or exhaust, wherebythe cyclic changing resistance oi the spark gap may be ascertained atthe said particular point.

8. A method of sistance of a spark plug installed in an operatinginternal combustion engine at a particular point in the operating cycleof its respective cylinder ascertaining the internal re I sistance of awhich comprises the steps oi applying a substantially constantelectrical potential to a series circuit including the spark gap of saidplug, applying at least part of the periodically developed ignitionpotential of another control spark plug installed in the same engine tothe control grid of a space discharge'device, discharging a chargedcondenser through the space discharge device when the said dischargedevice is rendered conductive by the periodically developed ignitionpotential applied to said grid, substantially immediately thereafterrecharging said condenser through a series circuit containing aresistance element, applying the potential drop across at least aportion of said resistance element to the electrodes oi a cathode raytube to establish said cathode ray, and applying the potential dropacross said gap to the deflecting plates of said cathode ray tubewhereby a rapidly recurring yet instantaneous measurement oi! the sparkplugs resistance at the particular point in the operating cycle of itsrespective cylinder may be obtained by noting the deflection of saidray.

' 9. A method of ascertaining the internal respark plug installed in anoperating internal combustion engine which comprises the steps ofconnecting a source of electrical current of substantially constantvoltage to a circuit including the spark gap of said spark plug,periodically initiating the flow of an electron beam at a particularpoint in the operating cycle of said internal combustion engine otherthan during ignition, combustion, or exhaust, and substantiallysimultaneous with the initiation of said g said beam an amountproportional to the voltage drop across said gap, whereby the locationof the beam spot will be the criterion of the resistance of said plug atthe particular point in the operating cycle of said internal combustionengine.

10. A method of determining the maximum resistance of a spark pluginstalled in an operating internal combustion engine'which includes thesteps of connecting a resistance to the said spark plug, connecting asubstantially constant potential supply source in circuit with saidresistance and the spark gap of said plug, and impressing the potentialvariations occurring in at least a portion of said resistance unitacross the deflecting plates 01' a cathode ray tube whereby thedeflection of said cathode ray will be a measure of the maximumresistance of said plug.

11. A method of ascertaining the resistance of a spark plug wheninstalled in an operating internal combustion engine which comprises thestep of impressing a known voltage across the spark gap of said 'plug ata particular point in of a test the operating cycle other than duringignition,

combustion or exhaust, and noting the current magnitude in the circuitincluding said spark gap, whereby the resistance may be ascertained atsaid particula r point.

particular point at which said resistance measurements are to be thesame engine taken, a control spark plug oi into which said test plug isinstalled, an ignition system for said spark plugs,

and means for a ignition potcntia trol grid of said urements are to betaken.

13. In a system for determining the resistance 14. In a system for of atest spark plug the resistance of determining the resistance installedin an operating gine, the combination in said spark plug, means forprothat point in the cylinder at which operating cycle oi its respectivethe electric beam is initiated. MELVILLE F.

JOHN P. BOSTON.

DAVIS.

